The present invention relates generally to an oil suppressing structure in an air drying device for suppressing oil contents or oil mist contained in compressed air which rises in an interior of the air drying device of particularly a vehicle compressed air brake system for realizing an improvement in quality. The present invention relates particularly to a dehumidifying and oil adsorbing structure in an air drying device of a vehicle compressed air brake system including a polyurethane filter portion which is provided in an interior of the air drying device for suppressing oil contents or oil mist contained in compressed air for realizing an improvement in quality.
Conventionally, there is a technique disclosed in U.S. 2004/0163535A1 as a first example of an air drying device of this type for dehumidifying and removing oil from air flowing into the system. This technique, shown in FIG. 10 of the present application, will be described below. Reference numeral 1 denotes an air drying device having an outer case 2 and a base plate 3 which is fixed to a lower end 2a of the outer case 2. The base plate 3 has inlet ports 3a through which compressed air, which still contains moisture, is allowed to flow into the air drying device 1 and an outlet port 3b which is opened in a substantially central portion thereof. The base plate 3 has, for example, a circular disk shape in whole which has a predetermined thickness. A cylindrical projecting body 3c is formed at the substantially central portion of the base plate 3 and the outlet port 3b is provided to extend through the cylindrical projecting body 3c so that dried compressed air is to be discharged therethrough.
Then, the base plate 3 has eight inlet ports 3a which are formed circumferentially in an outer circumferential portion of the outlet port 3b so as to be disposed at predetermined intervals, for example, at intervals of substantially 45°. A circular disk-shaped support plate 4 in which a number of openings 4a which are made up of minute holes is disposed in a substantially central position with respect to a height direction of the outer case 2 in an interior thereof. A circular cylindrical body 5 is formed in the interior of the outer case 2 so as to project upwards from the support plate 4. In addition, a circular cylindrical spring support body 6 is formed so as to be suspended downwards from the support plate 4. An element support member 7 is disposed on an outer circumference of the cylindrical spring support body 6. Generally speaking, the element support member 7 includes a collar portion 7a at an upper end portion thereof and a cylindrical portion 7b which is integrally formed thereon so as to extend downwards, and a spring receiving seat 7c is formed at a lower portion in the interior of the cylindrical portion 7b. A circular hole 7d is opened in a substantially central portion of the spring receiving seat 7c so as to communicate with the outlet port 3b in the base plate.
Then, a spring 8 which is placed in the cylindrical portion 7b is supported at a lower end and an upper end thereof by an upper surface of the spring receiving seat 7c and the cylindrical spring support body 6, respectively. On the other hand, an element gripping portion 7e is provided circumferentially at an outer circumferential edge of the color portion 7a of the element support member 7 so as to extend downwards. An element 9 is provided at the element gripping portion 7e, and a passage 10 is formed below the element 9, a compressed air check valve 11 being provided. In addition, a desiccating agent 12 is filled in a space defined above the support plate 4 in the interior of the outer case 2.
In FIG. 10, reference character S denotes a space defined between the outer case 2 and the cylindrical portion 7b of the element support member 7.
The air drying device 1 of the first example of the related art is configured in the way described heretofore. Thus, compressed air, which still contains moisture, flowing from a compressor (not shown) flows into the air drying device 1 from the inlet ports 3a and passes through the element 9 by way of the space S, whereby oil particles and moisture contained in the compressed air are removed. Then, the compressed air flows from the openings 4a in the support plate 4 into the desiccating agent 12. The compressed air is dehumidified and dried further by the desiccating agent 12 and is then allowed to flow into the cylindrical body 5 and through the interior of the cylindrical portion 7b of the element support member 7 so as to be discharged from the outlet port 3b. 
Next, there is a technique disclosed in U.S. 2006/0123743A1 as a second example of an air drying device, which is shown in FIG. 10. To describe this technique, reference numeral 1A denotes an air drying device, which has an outer case 2 and an inner case 2A. The inner case 2A is formed into a cylindrical shape, which has an upper large-diameter cylindrical body 2b and a lower small-diameter cylindrical body 2c. A multiplicity of discharge ports 2f are opened in a bottom plate 2e of the small-diameter body 2c, and the discharge ports 2f are made up of small holes which communicate with an outlet port 3b of a base plate 3. A filter plate 13 is laid over an upper surface of the bottom plate 2e. On the other hand, a first filter 14 is interposed between a lower surface of an outer circumferential edge of the bottom plate 2e and an upper surface of an outer circumferential edge of the base plate 3.
A circular cylindrical projection 2g is provided on a lower surface of a substantially central portion of the bottom plate 2e of the inner case 2A so as to project downwards therefrom. A distal end of this circular cylindrical projection 2g is fixed to an outer circumference of a cylindrical projecting body 3c formed at the substantially central portion of the base plate 3 so as to project upwards with a seal material 2h interposed between the circular cylindrical projection 2g and the cylindrical projecting body 3c. An air flowing passage S1 is formed between an outer circumferential surface of the large-diameter cylindrical body 2b and an inner wall surface of the outer case 2, and a bypass passage S2 is formed between an outer circumferential surface of the small-diameter cylindrical body 2c of the inner case 2A and the inner wall surface of the outer case 2.
Reference numeral 15 denotes a second filter, which is fixedly disposed between a lower surface of an outer circumferential edge of the large-diameter cylindrical body 2b of the inner case 2A and a lower end edge of the small-diameter cylindrical body 2c within the bypass passage S2. An exterior cover 17 including a seal member 16 provided on a lower surface is fixed to a lower portion of the base plate 3. Inlet ports 17a which communicate with inlet ports 3a of the base plate 3 and an outlet ort 17b which communicates with an outlet port 3b of the base plate 3 are formed in the exterior cover 17. In FIG. 11, reference numeral 2d denotes a lid plate of the inner case 2A, and a cylindrical body 2j is provided at a central portion of the lid plate 2d so as to project upwards and a number of openings 2k, . . . are opened in the lid plate 2d. A filter plate 18 is disposed on a lower surface of the lid plate 2d, so as to confine a desiccating agent 12 filled within the inner case 2A therein. Reference numeral 19 denotes a spring, which is placed within the cylindrical body 2j and is supported by an upper wall surface 2i. 
In the figure, reference character V denotes a check valve, which is disposed in the bypass passage S2. The air drying device 1A of the second example of the related art is configured in the way described above. Thus, compressed air, which still contains moisture, flowing from a compressor (not shown) flows into the air drying device 1A from in inlet ports 3a, passes through the second filter 15 and the air flowing passage S1 by way of the first filter 14 and the bypass passage S2, and flows into the desiccating agent 12 by way of the openings 2k in the lid plate 2d. Then, the compressed air is dehumidified and dried further by the desiccating agent 12, flows through the discharge ports 2f in the base plate 2e by way of the filter plate 13 and is discharged from the outlet port 3b of the base plate 3.
When the compressor compresses outside air taken thereinto from the outside, oil mist of the compressor is mixed into compressed air. In general, in an air drying device, oil mist is removed from compressed air by a filter before water contents are removed from the compressed air. However, in the air drying devices of the first and second examples of the related art, oil mist cannot be removed sufficiently by the filter or the like, and hence, the desiccating agent is contaminated with oil mist. As a result, the drying of compressed air becomes insufficient, whereby the corrosion of various pneumatic devices and equipment installed in a vehicle including a brake system is promoted by water contents in the compressed air. In particular, there has been caused a problem that the vehicle becomes unable to run abruptly due to a failure in the brake system. In addition, in winter and cold regions, there has been caused a problem that water contents or droplets of water which enter various pneumatic devices and equipment freeze these various pneumatic devices and equipment, leading to the generation of operation failure. Further, oil from the compressor comes to stick to the filter element and the periphery thereof to thereby be accumulated and overflows from the filter element. Then, the oil flows through the desiccating agent into the various pneumatic devices and equipment so as to deteriorate rubber products such as seal materials and packings, resulting in a problem that the pneumatic devices and equipment are caused to fail by the deterioration of such rubber projects.